Aetiology or causes

The pressure ulcers are caused by impaired blood supply and tissue malnutrition, as a result of prolonged pressure, friction, or shear. The development occurs with the disruption of the vascular network of arteries, arterioles and capillaries [28]. Tissue compression exceeding the capillary filling pressure of 32 mmHg, that lasts longer than 2 hours, can cause local ischemia and necrosis. Skin overlying bony prominences (eg, sacrum, malleoli, or hips) are especially vulnerable [29].

Carol Diane Hepburn 20 | P a g e

As regards the pressure, the intensity, duration, and the tissue's tolerance for pressure, must be considered [30]. Not only the intense pressure, but also moderate pressure acting continuously over a long period of time can result in ulcer formation [30]. A theory was put forward, that the duration of the pressure was more important than the amount of pressure sustained by the capillaries [31]. This being the case, the pressure is one of the most important factors in the formation of pressure ulcers, but not limited to only this component or factor [30, 32, 33, 34].

There are different types of pressure which can cause a pressure ulcer and these are:

 Interface pressure – which occurs with the pressure of the body pressing the

skin down onto a firm surface, which can be either, a bed, a wheelchair, or cushion etc.

 Shear – this is pressure which occurs, when layers of skin are forced to slide

over one another, or deeper layers of tissue slide over one another. Shear can occur when a person slides down or is pulled up, out of a bed or out of a wheelchair

 Friction– this pressure is caused by something rubbing against the surface of

the skin, such as a mattress, cushion or clothing [35].

In an alert person, the body’s motor and sensory systems are responsible for relieving the effects of a continuous load or pressure, which usually results in the initiation of frequent small body movements and periodic changes to their posture to relieve the load and restore tissue perfusion [36]. This form of subconscious postural shifts or fidgeting ensures that we move when needed.

Many people at risk of developing pressure ulcers are either, unable to effectively reposition themselves, or are not provided with the sensory feedback that prompts the required subconscious movements. In the case of patients who are unconscious, sedated, anaesthetised, have limited mobility, or are paralysed and therefore cannot sense or respond to these signals, cannot initiate these spontaneous movements. This can result in the skin and soft tissues being subjected to, prolonged and unrelieved pressures if no intervention is put in place. Therefore, people with medical condition that limits their ability to change positions, requires them to use a wheelchair or confines them to a bed for long periods of time are most at risk of pressure ulcers [37].

Carol Diane Hepburn 21 | P a g e

angle to a surface. Where the same amount of force is applied to both a small and large area, a greater pressure is exerted on the smaller area than the larger, see Figure 2.13 [38, 39].

Figure 2.13. Diagrammatic definition of pressure [38, 39]

When the pressure over the bony prominences distorts the skin and underlying tissues,

internal stresses have also been found to be ‘tensile’ due to stretching and ‘shear’

caused by the distortions. This can mean that even when pressure is applied perpendicular, tensile and shear can also occur within the underlying soft tissues near bony prominences, see Figure 2.14 [38, 39, 40]. The MRI (Magnetic resonance imaging) studies shown in Figure 2.15 also illustrates significant distortion during loading [41]. The pressure exhibited at the intersection between the skin (or skin & clothing) and a support surface (bed, mattress, wheelchair, wheelchair cushion) is sometimes called the 'interface pressure'.

Carol Diane Hepburn 22 | P a g e

Figure 2.14. Tissue distortion due to pressure.

Bending of the lines in (b) shows that when external pressure is applied over a bony prominence, compressive, shear (distorting) and tensile

(stretching) stresses occur. [38, 40]

Figure 2.15. MRI showing the effect on the tissues during the application & non- application of load during sitting. [41]

Carol Diane Hepburn 23 | P a g e

Shear stress is the application force parallel or tangential to the surface of an object with the base of the object remaining stationary. This causes the object to change shape or deform. Shear stress is expressed in similar terms as pressure, most

frequently as pascals (Pa), or sometimes as newtons/square metre (N/m2).

Shear stress = Tangential force applied (N)

(pascals or Area of application of force (m2)

N/m2)

1Pa = 1N/m2 1kPa = 1000N/m2 --- Equation 2.1

The effect of shear stress on the internal tissues can be seen in Figure 2.16 [38, 42]. With the application of shear force, friction between the skin and the support surface has the tendency to hold the skin in place, while deeper tissues are displaced. The amount of displacement, i.e. shear strain, is greater in the vicinity of the bone than in the superficial tissue layers.

Figure 2.16. Effect of shear stress on body tissue layers [38, 42]. .

The areas of greatest shear stress are near the bony prominences, where interface pressures tend to be the highest. This can mean patients with slender body types have a tendency to have higher shear stress in the coccyx and sacrum, than do obese body types. This effect prevents the blood flow within the blood vessels in these areas, by several mechanisms, such as; direct compression and occlusion of blood vessels,

stretching and narrowing of the dermal capillary beds – when sufficiently high shear

stresses are applied, the internal diameter of the capillaries becomes inadequate for blood flow [43, 44]; bending and pinching the blood vessels running perpendicular to the skin surface [38, 45].

Carol Diane Hepburn 24 | P a g e

2.2.5.3. Friction

Friction is the force that counters the relative motion of two objects that are touching and is measured in Newtons (N). However, the term 'friction' as mentioned before, is also frequently used to mean the action of one object rubbing against the other. Friction helps in the development of shear stresses with the human body, by keeping the skin in place against a support surface while the rest of the body moves towards the foot of a bed or the edge of a seat. The relative movement of the skin and underlying tissues causes shear stresses to develop in the soft tissues overlying the bony prominences such as the sacrum [38, 39].

2.2.5.4. Additional causes for pressure ulcers

As mentioned previously the pressure is not the only factor in the causes of pressure ulcers. Other extrinsic or external factors, in addition to pressure, friction and shear, are moisture and heat (sometimes encompassed as the microclimate). The intrinsic factors are, reduced mobility, impaired sensation, acute, chronic or terminal illness, pyrexia (high temperature), dehydration, incontinence/other moisture sources, vascular disease, malnutrition, a history of pressure ulcers, pain effecting the desire to reposition themselves, some types of medication (e.g. steroids), old age, levels of consciousness and cognitive status [46, 47]. This list is not exhaustive.

2.2.5.4.1 The microclimate

The microclimate, identified as the environment near the interface with skin/clothing and the support surface, has a direct relationship with some of the extrinsic factors, such as heat and moisture. Many studies have stated that the microclimate includes the skin temperature and skin moisture between the patient’s interface with the skin and the support surface, this sometimes includes air movement.

An increase in skin moisture contributes to a series of damaging forces, such as maceration leading to skin breakdown, it weakens the stratum corneum (outer layer of the epidermis or skin), leading to skin damage [48, 49, 50]. Equally, excessive dryness can lead to skin damage by cracking. A reduction in skin resilience and an increase in the skin’s coefficient of friction due to skin moisture from increased perspiration, will also give rise to an increase in shear stresses and friction, making the individual prone to pressure ulcers. When the skin temperature is above approximately 33°C, local

Carol Diane Hepburn 25 | P a g e

also giving raise to the ideal environment for the development of a pressure ulcer. Other factors that can increase excessive moisture on the skin surface as well as perspiration are, urinary or faecal incontinence, wound/fistula drainage or vomit. All these components increase the risk of pressure ulcerations by weakening the cross linkages between the collagen and damaging the epidermis.

Another significant risk factor for pressure ulcers is an increased body temperature (pyrexia). An increased skin temperature is related to pressure ulceration by increasing susceptibility to the ischaemic effects of the pressure and shear stresses and by weakening the stratum corneum.

Whether a support surface will have an impact on the microclimate will depend on the characteristics of the support surface. For materials, such as foam, the surfaces have poor heat transfer properties, gel-filled products have a cooling effect, however, these wear off after time exceeding 2 hours [38] and can increase the humidity in that area. Fluid-filled products and alternating pressure air mattresses, both reduce skin temperatures [38, 50, 51]. Fabrics, such as 3D knitted spacer fabrics have the properties which dissipate heat and help to evaporate moisture [52, 53, 54, 84].